Hard magnetic disks are used to store digital information utilized for data processing. An advantage of such a disk is that it can provide high-speed random access. That is, one can either write or retrieve information from any selected area on the magnetic memory surface without having to serially traverse the full memory space of the disk. Generally, a hard magnetic disk is mounted within a disk drive which is akin to a record turntable in that it includes means for rotation of the disk and means for translating a head across the surface of the disk to provide access to a selected annular track. Typically, a plurality of disks, such as two or four, are mounted on a single spindle in spaced relationship to one another and heads are provided to interact through their magnetic fields with oppositely close planar surfaces of each of such disks.
The disks now available for hard disk memory applications are typically magnetic. Each of the disk surfaces which receives and stores information has a thin layer of magnetic material carried by a substrate. The heads which interact with each of the surfaces are so-called "flying" heads i.e., they do not touch the surface of the disk during its rotation-rather, they ride on an air film which acts as a bearing between the disk and the head. The head typically includes a magnetic coil to permit interacting with the magnetic film through the intervening air film space. The air film prevents wear of the head and the thin magnetic layer on the disk surface which would otherwise be caused by a contact between the head and the surface film.
Thin film magnetic media are built up in layers, each of which performs a specific task. As shown in FIG. 1, the basis metal of the disk is generally an aluminum alloy, typically 0.030 inches thick for 2 inch diameter disks and 0.075 inches thick for 5.25 inch diameter disks. Disk alloys generally contain about 4.5 to 5 percent magnesium to add strength to the disk. Because these alloys are soft, a hard surface is built up by adding a coating of nickel-phosphorous (Ni--P) alloy by the immersion process known as electroless nickel plating. The Ni--P layer is typically 300 to 400 microinches in thickness, after polishing to obtain a smooth surface. The hard Ni--P layer is a firm base which provides support to much thinner subsequently added layers. The Ni--P resists mechanical damage which might be caused by inadvertent contact impacts between the head and disk surface.
The major steps of coating a disk with the several layers necessary for a thin film memory disk in accordance with the prior art are shown in FIG. 2. The aluminum alloy substrate (disk) is degreased by organic solvents such as Freon, trichlorethylene or isopropyl alcohol. It is then washed in an alkaline soap solution and then rinsed in water. It is then etched in a dilute acid bath generally containing hydrochloric, hydrofluoric and nitric acids and then rinsed. The surface is then prepared for electroless nickel plating of the Ni--P layer by a double zincating process. The double zincate immersion serves a fourfold purpose. Firstly, in the first zincate immersion, the concentrated alkaline solution, such as sodium hydroxide solution, chemically dissolves aluminum oxide from the surface of the aluminum alloy disk. Otherwise, the thin insulating oxide layer found naturally on aluminum would prevent the growth of Ni--P. Secondly, also in the first zincate immersion, zinc electrochemically replaces the aluminum freshly bared by dissolving away the thin oxide layer. A small amount of aluminum metal dissolves and is immediately replaced by a deposit of zinc.
The disk is then rinsed in water and then placed in a nitric acid solution which removes the first zinc deposit, leaving a fully activated surface. The disk is rinsed in water and is then again immersed in an alkaline zincate solution. The surface becomes more completely covered by zinc in the second zincate immersion.
A third function of the zinc layer is as a protective coating. The zinc protects against oxidation of the aluminum alloy disk surface during the following rinsing and during transit to the electroless nickel tank. The disk is rinsed in distilled water and placed in an electroless nickel plating solution to grow a Ni--P protective film.
The fourth action of the zinc coating is the important step of getting the Ni--P plating started. As the zincated surface is immersed in the nickel solution, the zinc provides an automatic battery action to trigger Ni--P deposition. The zinc dissolves into the solution, exposing clean aluminum, and is instantly replaced by a deposit of Ni--P. The deposited Ni--P bonds tightly with the oxide-free aluminum alloy substrate. Once initiated, the growth of Ni--P is self-nucleating as each newly added layer generates further Ni--P deposition.
The zinc coating is strongly affected by the metallurgical microstructure of the aluminum alloy. In the first zincate immersion, zinc is deposited at isolated centers. Although acid dipping and the second zincate immersion produce a more uniform coating, isolated centers of above-average chemical activity remain. These centers are associated with intermetallic compounds formed by impurities in the alloy.
For memory disks, a commonly used aluminum alloy has been 5086, with 4.0 weight percent magnesium as the principal strengthening agent. The microstructure of rolled 5086 alloy sheet contains a large number of inclusions, typically ranging up to one micrometer in size. The inclusions are various intermetallic compounds such as Al.sub.12 (FeMn.sub.3) Si formed from the combination of aluminum with iron, silicon and other impurity elements. Thus, even though the impurity elements are generally present in amounts typically less than 0.1 percent by weight, they combine with aluminum to form compounds which occupy 1 to 5 percent of the disk surface area and are thus available to participate in the reactions of the wet chemistry.
Newer aluminum alloys such as CZ46, 5186 and 5586 have been developed with fewer and smaller intermetallic inclusions in comparison with the 5086 composition. While this approach has been successful, the task of double zincating remains that of compensating for the localized variations in disk surface chemistry due to the presence of the intermetallic particles. Hydrogen gas evolution can occur preferentially on intermetallic particles. Pits in the Ni--P deposit can result when adhering gas bubbles block off Ni--P plating. Intermetallic particles also serve as preferential sites of Ni--P deposition. As a result, localized growth from such a site can be ahead of the general linear advance of Ni--P growth. The advanced localized growth nucleates additional radial growth, ultimately becoming a large hemispherical nodule on the final deposit surface. Thus, the imperfections in the initial layer can cause growth (nodules) or absence of growth (pits) which thereafter propagate through the following deposits.
The Ni--P layer is plated extra thick by the electroless method and the rough nodular surface is then partially polished away so that a completely dense and smooth surface remains. The smooth surface is then lightly textured to aid flying of the head. The disks are then loaded into a vacuum sputtering system indicated by the dotted box, FIG. 2. As the disk surface passes by several stations within the vacuum chamber, layers are built up by material selectively deposited by vacuum sputtering from targets on each side of the in-line vacuum sputtering system. The first in-line station preheats the disk surfaces, followed by a station for reverse sputter etching to remove any accumulated thin oxide layer from the Ni--P. Layers of chromium, cobalt alloy and carbon are then sputter deposited from a sequence of sputter targets, all within the same vacuum chamber.
The first vacuum sputtered layer to be deposited is chromium, typically from about 0.4 to 4 millionths of an inch thick (0.4 to 4 microinches), as shown in FIG. 1. The chromium (Cr) beneficially orients the crystal growth of the next vacuum sputter deposited layer, an alloy in which cobalt (Co) is the major constituent. The magnetic cobalt alloy is the key layer for information storage and is typically vacuum sputter deposited to a thickness of about 2 microinches (500 .ANG.).
A carbon overcoat layer is then vacuum sputter deposited on top of the magnetic layer, with a thin (0.2 microinch) intermediate layer of sputtered chromium being first added to bind the carbon to the magnetic layer. The carbon overcoat is generally vacuum sputter deposited to a thickness of about 1 to 2 microinches, as shown in FIG. 1. The vacuum sputter deposited carbon layer is a low friction surface upon which the head can slide, when required. The lubrication property of the vacuum sputter deposited carbon layer is usually enhanced by the later addition of a thin fluorocarbon coating outside of the vacuum sputtering chamber. As a final step before packaging and shipment, each disk is certified by carefully measuring its response to standard read-write signal patterns.
Although the Ni--P layer performs its mechanical task well, several wet chemical surface preparation steps are needed to assure good adhesion of the Ni--P on the aluminum substrate. The many steps of the double zincate treatment present undesirable opportunities for mishaps and correspondingly low yields.